Many museums are using Flickr groups to help promote their collections. I love that some of the big museums in the UK (e.g. Science, British Museum, Natural History, V&A) seem to have a free for all approach to photography, that is generally allowed unless expressly forbidden. The V&A in particular has taken an active approach to managing images from their museum, inviting images to its Flickr group.
Smaller museums and those supported or run by local authorities actually seem to take a more controlled approach to photography, probably missing a trick in terms of promotion of its collections and special exhibitions.
Walsall Museums has recently held an event where it not only opened up its collection to local photographers (those already sharing their photos in a Walsall Flickr group), but opened up its stores as well. Dan Slee of Walsall Council outlines the agreements reached on copyright of the images and access on his blog. This is cool, because it's not only sharing images of the collection, but reaching the bits that most people never get to see.